Need to explain how light propagates at a given wavelength with a certain phase velocity that depends on the refractive indices of the core and the cladding, as well as on the diameter of the core.

Explain how light propagates at a given wavelength with a certain phase velocity that depends on the refractive indices of the core and the cladding, as well as on the diameter of the core. Based on this phase velocity, an “effective index” for the lightwave can be defined. This effective index is the parameter that relates the peak wavelength of the reflection spectrum and the period of the grating.